Wood fibre based direct pressed laminated flooring usually comprises a core of a 6-12 mm fibre board, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like material.
A laminate surface generally comprise two paper sheets, a 0.1 mm thick printed decorative paper and a transparent 0.05-0.1 mm thick overlay paper applied over the decorative paper and intended to protect the decorative paper from abrasion. The print on the decorative non-transparent paper is only some 0.01 mm thick. The transparent overlay, which is made of refined α-cellulose fibres, comprises small hard and transparent aluminium oxide particles. The refined fibres are rather long, about 2-5 mm and this gives the overlay paper the required strength. In order to obtain the transparency, all natural resins that are present in the virgin wood fibres, have been removed and the aluminium oxide particles are applies as a very thin layer over the decorative paper. The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers one over the other.
The printed decorative paper and the overlay are impregnated with melamine resin and laminated to a wood fibre based core under heat and pressure.
The small aluminium oxide particles could have a size in the range of 20-100 microns. The particles could be incorporated in the surface layer in several ways. For example they could be incorporated in the pulp during the manufacturing of the overlay paper. They could also be sprinkled on the wet lacquer during impregnation procedure of the overlay or incorporated in the lacquer used for impregnation of the overlay.
The wear layer could also be produced without a cellulose overlay. In such a case melamine resin and aluminium oxide particles are applied as a lacquered layer directly on the decorative paper with similar methods as described above. Such a wear layer is generally referred to as liquid overlay.
With this production method a very wear resistance surface could be obtained and this type of surface is mainly used in laminate floorings but it could also be used in furniture components and similar applications. High quality laminate floorings have a wear resistance of 4000-6000 revolutions, which corresponds to the abrasion classes AC4 and AC5 measured with a Taber Abraser according to ISO-standard.
It is also known that the wear resistance of a lacquered wood surface could be improved considerably by incorporating aluminium oxide particles in the transparent lacquer covering the wood surface.
The most common core material used in laminate floorings is fibreboard with high density and good stability usually called HDF—High Density Fibreboard. Sometimes also MDF—Medium Density Fibreboard—is used as core. Other core materials such as particleboard are also used.
HDF is produced as follows: Roundwood such as for example pine, larch or spruce are reduced to wood chips and then broken down into fibres in a refiner. The fibres are thereafter mixed with a binder and then subjected to high pressure and temperature to form a board.